feat: add comprehensive unit tests for bittensor.core.threadpool module

[codomposer]

Pull Request: Add Comprehensive Unit Tests for ThreadPool Module

Bug

Missing test coverage for critical infrastructure component bittensor/core/threadpool.py (10.5KB). The PriorityThreadPoolExecutor had no dedicated unit tests, leaving critical concurrent task execution infrastructure untested.

Description of the Change

This PR adds a comprehensive test suite (tests/unit_tests/test_threadpool.py) with 35 unit tests covering all aspects of the threadpool module:

Test Coverage Added:

Core Functionality (9 tests)

• WorkItem creation, execution, exception handling, and cancellation
• Stale task detection based on BLOCKTIME
• Executor initialization with various parameters
• Input validation (invalid max_workers, non-callable initializers)
• Queue state monitoring (is_empty property)

Task Submission & Priority Management (5 tests)

• Task submission with various priority levels
• Priority queue ordering verification
• Random priority assignment for tasks without explicit priority
• Epsilon randomization for priority tie-breaking
• Thread-safe concurrent task submission

Thread Pool Scaling & Lifecycle (2 tests)

• Thread creation up to max_workers limit
• Worker thread creation, execution, and termination

Shutdown Behavior (3 tests)

• Graceful shutdown (wait=True)
• Immediate shutdown (wait=False)
• RuntimeError on post-shutdown submissions

Error Handling (2 tests)

• BrokenThreadPool exception handling
• Submission to broken pool validation

Initializer Functions (2 tests)

• Custom initializer with initargs
• Failed initializer handling and pool marking as broken

Configuration & Environment (5 tests)

• BT_PRIORITY_MAX_WORKERS environment variable
• BT_PRIORITY_MAXSIZE environment variable
• Argparse integration (add_args method)
• Custom prefix support
• Config class method validation

Worker Function Behavior (2 tests)

• Worker exit on NULL_ENTRY signal
• Work item execution from queue

Edge Cases (5 tests)

• Priority=0 conversion to random value
• Multiple shutdown calls safety
• Context manager compatibility
• Large number of tasks (100 tasks stress test)
• Mixed args and kwargs handling

Alternate Designs

Alternative approaches considered:

1. Integration tests only - Rejected because unit tests provide faster feedback and better isolation
2. Mocking the entire executor - Rejected because we need to test actual concurrent behavior
3. Fewer tests with broader scope - Rejected to ensure comprehensive coverage of all code paths

The chosen approach provides thorough unit-level testing while maintaining test independence and fast execution.

Possible Drawbacks

• Adds ~700 lines of test code to maintain
• Some tests use time.sleep() which may be flaky in extremely slow CI environments
• Thread-based tests may occasionally have timing issues, though timeouts are generous (2-5 seconds)

Results:

35 passed, 1 warning in 4.01s

All tests pass successfully:

• ✅ 4 WorkItem tests
• ✅ 5 PriorityThreadPoolExecutor initialization tests
• ✅ 5 Task submission tests
• ✅ 2 Thread pool scaling tests
• ✅ 3 Executor shutdown tests
• ✅ 2 BrokenThreadPool tests
• ✅ 2 Initializer tests
• ✅ 5 Configuration tests
• ✅ 2 Worker function tests
• ✅ 5 Edge case tests

Test Categories Verified:

• ✅ Concurrent task execution
• ✅ Priority-based ordering
• ✅ Thread safety
• ✅ Error handling
• ✅ Configuration management
• ✅ Resource cleanup
• ✅ Edge cases

Contribution by Gittensor, learn more at https://gittensor.io/